This invention relates to projection color television (TV) display devices using monochrome cathode ray tubes (CRTs) incorporating interference filters, and more particularly relates to a method for improving the white field uniformity of these devices, and to the resulting devices and tubes.
Monochrome cathode ray tubes for projection television employ a single electron gun mounted in the neck of the tube to focus a single electron beam on the fluorescent display screen of the tube. A deflection yoke surrounding the neck of the tube, and associated electronic circuitry, cause the beam to scan the screen as well as to vary in intensity in response to a video signal to produce a monochrome display image.
In projection color television, three such displays, each in one of the primary colors red, blue and green, are superimposed on a large projection screen to produce a full color display image. Because the images on the individual tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher loads than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness.
Projection tubes having an interference filter are described in U.S. Pat. No. 4,634,926, assigned to U.S. Philips Corporation. The filter, herein referred to as a short wave pass (SWP) filter, is composed of alternating layers of materials of high and low refractive index. The filter is designed to result in a marked increase in luminous efficiency of the tube in the forward direction, as well as improved chromaticity and contrast. Even further improvements are provided, especially in light gain in the corners of the display screen, by combining such an interference filter with an inwardly curved display window, as provided in U.S. Pat. No. 4,683,398, also assigned to U.S. Philips Corporation.
Such tubes with interference filters, while exhibiting a marked increase in luminous efficiency in the forward direction, as well as improved chromaticity and contrast, also exhibit greater luminance in the center than at the edges of the display, referred to herein as center-to-edge luminance gradient, particularly along the minor or y axis of the tube.
Furthermore, uncontrollable variations in the thicknesses of the filters occurring during mass production of these tubes can result in edge luminances which vary from tube to tube from as low as about 30 to as high as about 70 percent of the center luminance. When the red, blue and green tubes are assembled into a projection color television, the tube drivers are adjusted to result in a desired white field at the center of the screen. It can be appreciated that when red, blue and green tubes having different center-to-edge gradients are combined in a projection set, white field uniformity is increasingly degraded as the distance from the center of the screen increases.
In co-pending patent application Ser. No. 289,388, filed concurrently herewith, the white field uniformity of such a projection color television display device employing at least one monochrome tube with an interference filter is improved by altering the center-to-edge luminance gradient of at least one of the projection tubes to match that of at least one of the other tubes. For example, the luminance gradient of the red tube is altered to match that of the green tube, such as by applying an optical filter to the outside surface of the display window, the filter having varying transmission from center to edge. Since the uncontrollable thickness variations result in a range of center-to-edge gradients for the red and/or blue tube, custom matching on a tube-for-tube basis is required.